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As with their choice of white cotton, most clothing manufacturers prefer synthetic dyes because they are cheaper and easier to use.

However, there has been growing concern about the impact of the colouring industry on human health and on the environment, particularly when waste water from the process is often discharged with inadequate or no treatment.

While the use of harmful chemicals has drawn the most attention, green groups argue we should be conscious of the whole process. The National Resources Defense Council (NRDC), an influential American NGO, identifies the impact of dye mills from three main factors: discharge of untreated or poorly treated effluents, continued use of hazardous colouring agents and poor plant management resulting in a waste of water, energy and chemicals.

"When you're working on something for the environment, it's not just what's in the dyes, but also how you treat the effluent from the dyeing process," she says.

Lemmlie-Leung launched the Hong Kong Organic Textile Association last year to counter widespread misinformation in the marketplace about organic clothing.

"A lot of people think if [a fabric] is labelled organic, it just means it's soft. It can be soft, of course, but that's because of the type of yarn or the way it was manufactured," she says.

The value of organic materials lies in the fact that the cotton or other plant fibre used is grown without pesticides, Lemmlie-Leung says, so it's better for the environment and farming communities.

Such textiles steer clear of synthetic dyes that use heavy metals and toxic chemicals as catalysts for dyeing and fixing the colour.

That is not to say simply using natural dyes would improve the environmental impact.

"People think natural is safe, but not necessarily so," Lemmlie-Leung says. "There are natural substances that can hurt you or the environment. For example, potato starch is used [in textiles], but if you dump that into a river, it will kill the fish."

That is why, the NDRC notes, if recycled, organic fabric were dyed in a poorly managed plant, the process would probably cause more harm to the environment than a conventional fabric dyed in a well-managed factory.

There is a lot more to living up to the organic label, says Lemmlie-Leung, whose factory was the first terry-weaving mill on the mainland to qualify for Global Organic Textile Standard certification in 2009. This includes using dyes that cause less damage to the environment. "Generally, low-impact dyes are fibre-reactive and do not contain toxic chemicals that fix the dye to the fabric. They also have a higher absorption rate and require less rinse water," she explains.

The association hopes to help people understand what the standards mean and to know when to look for those certifications when they shop, she says.

This article appeared in the South China Morning Post print edition as: